Only a portion of the oil present in an oil-bearing subterranean formation is recoverable as a result of the natural pressure of the formation. The oil recovered from this “primary” recovery typically ranges from 5% to 35% of the oil in the formation. The oil that remains is trapped by capillary forces in the pore space of the oil-bearing formation, or is attracted to the formation through electrostatic or Vander Waals attraction, and will not easily flow to a production well.
It is well known that hydrocarbon bearing subterranean formations may be stimulated to initiate or to increase the production of hydrocarbon liquids and gases from the formation. Some of the methods for stimulation involve injecting water or an aqueous solution into the formation to facilitate hydrocarbon flow into a production well, through which the hydrocarbons are “produced” to the surface for processing and use. Secondary recovery methods include injecting fluids into the formation to increase formation pressure, and to displace hydrocarbons from the formation to production facilities. Waterflood fluids introduce an immiscible phase into the formation to drive oil in the formation to production wells. Gases such as natural gas, air and carbon dioxide which may be injected into the formation for secondary recovery decrease the density of the oil in the reservoir to improve oil flow through the formation and through the production well.
Enhanced, or tertiary oil recovery methods increase the mobility of the oil in order to increase extraction. Thermal methods may be employed to increase the formation temperature and decrease the oil viscosity. Thermal methods may also create or enhance porosity in the formation to facilitate the flow of oil to a production well. Steam injection and fire flooding are two thermal methods that have been successfully employed to increase oil production from heavy oil reserves. Using water containing a surfactant and optionally one or more polymers as a liquid flood may also be used. The surfactant is believed to reduce the surface tension between the water and the oil in the reservoir. Using water as a viscous drive to displace oil in the formation is substantially improved as an enhanced oil recovery method by the reduced surface tension between water and oil in the formation.
But conventional enhanced oil recovery methods fail to recover all of the available oil in many hydrocarbon bearing formations. One approach has been to modify a waterflood fluid in order to effect changes in the wettability of the inorganic matrix/crude oil interface in the pore spaces of the formation. Many different variations on this approach have been disclosed, with they appear to be limited to specific reservoirs. The principles cannot be applied generally across the oil production space.
Additional recovery tools are needed for recovering the residual oil remaining in capillaries and small cavities in the inorganic matrix of the formation. To dislodge this residual oil, an aqueous displacement fluid must include chemical properties for changing the wettability of the inorganic matrix in contact with oil and brine, or the clay that naturally resides on the inorganic matrix surface, and between the oil adsorbed to the surface and the surface itself. Much recent work has been directed to methods for effectively modifying the wettability of the inorganic matrix to increase oil production.
U.S. Pat. No. 5,148,705 describes a method and formation test tools for making in situ measurements in a borehole to determine wettability of a formation, particularly in zones of irreducible water saturation. Wettability estimation can also be accomplished using the rock surface composition obtained by XPS (x-ray photoelectron scattering), and correlating it with the amount of organic carbon absorbed on the surface. Accordingly, the entire disclosure of U.S. Pat. No. 5,148,705 is incorporated herein by reference.
But the process of extracting the trapped oil from cavities and capillaries in the formation involves several mechanisms, including reducing the oil/rock attraction in the capillaries and then displacing the oil droplets from the cavities without breaking the droplets in such a way that only a fraction of the oil in the capillaries is removed and recovered. Most recently published methods for recovering oil have focused on the ionic profile of the waterflood fluid (e.g. US20140041856, U.S. Pat. No. 7,987,907 and US20120143579). These methods have not fully addressed the multiple processes involved in removing the remaining oil from the oil-bearing formation. Additional developments are necessary to improve the effectiveness of tertiary oil recovery methods.